Invasive direct blood pressure measurement is often performed on patients in the intensive care unit or the coronary care unit of a hospital, during heart surgery, and so forth. This method can observe the blood pressure waveform with a higher level of accuracy than a non-invasive indirect blood pressure transducer, and can also measure a local pressure such as an internal pressure of the heart. The following three systems have been employed for this purpose and are examples of invasive direct blood pressure transducers. FIGS. 10 and 11, discussed below, are explanatory views of conventional systems. Hereinafter, these three systems and their drawbacks will be explained.
(1) External Sensor System Using Semiconductor Pressure Transducer
This first system is illustrated in FIG. 10 and is the most widely used type at present. The transducers of this type have been put on the market by a large number of manufacturers. A catheter 1 filled with a physiological saline solution through a pressure tube 2 is inserted into a blood vessel of the arm of a patient, and the other end of the pressure tube 2 is connected to a semiconductor pressure transducer 3. This transducer 3 detects the change of the pressure of the saline solution, and a monitor 4 observes the change of the blood pressure.
This system is convenient and can accurately measure a patient's blood pressure for an extended period of time. Since many of the transducers are of a disposable type, there is a less danger of infection to others. Nonetheless, since the semiconductor pressure transducer 3 is of an electrical type, there is the danger of electric shock to the patient due to the failure of the apparatus, and the like. Furthermore, since the semiconductor pressure transducers 3 are complicated in structure, the problem of the high production cost is left unsolved when using them as a disposable device.
(2) Sensor System Using Catheter with Semiconductor Pressure Transducer at Its Tip
This second system is shown in FIG. 11. A small semiconductor pressure transducer 5 is assembled into the tip of a catheter 1, and the pressure of a local portion (e.g. internal chamber of the heart, large blood vessels, etc) is observed by a monitor unit 4 by inserting the catheter 1 into the blood vessel or chamber. The physiological saline solution is charged into a pressure tube 2 in the same way as in the first system. This system, too, has already been put on the market.
According to this system, however, it is difficult to produce a probe having as small a diameter as the catheter 1. Furthermore, this system is not suitable for taking measurements over a prolonged time, e.g., by leaving the probe inside a small blood vessel. Finally, the danger of electric shock to the patient is even greater than in the first system described above.
(3) Sensor System Using Catheter with Optical Fiber
This third system has the same basic structure as the second system. However, to eliminate the potential problem of electric shock inherent in the second system, this system uses a sensor comprising the combination of an optical fiber and a diaphragm, etc, in place of the semiconductor pressure sensor 5 (see KOKAI Japanese Unexamined Patent Publication No. 62-47335).
While the problem of harmful electric shock can be avoided by using this optical system, other problems remain. For example, additional components such as a diaphragm, etc., become necessary, therefore, the sensor becomes larger. This system is also unsuitable for taking measurements over a prolonged time.